The present invention relates to a process for the purification of norbornene by distillation.
The synthesis of norbornene from dicyclopentadiene (DCPD) or cyclopentadiene (CPD) and ethylene gives a crude norbornene comprising impurities such as DCPD, dimethanooctahydronaphthalene (DMON), CPD and the like (these impurities will be described in more detail hereinbelow). The separation of these impurities from the norbornene is complicated because of the numerous equilibrium reactions existing between these products: any distillation of a crude synthetic product results in a reactive distillation.
U.S. Pat. No. 3,007,977 discloses a process for the purification of norbornene from a reaction mixture obtained by reaction of ethylene with CPD and DCPD, the said reaction mixture comprising unreacted CPD and norbornene. This process comprises the stages consisting in:
in a first fractionation region, fractionating the said reaction mixture in order to obtain, as intermediate fraction, norbornene with minor amounts of CPD and to discharge the DCPD from the said fractionation region;
in a second fractionation region, fractionating the said intermediate fraction into a top fraction comprising CPD and norbornene and into a fraction comprising essentially norbornene.
In accordance with this U.S. Pat. No. 3,007,977, recourse is thus had, for the purification of a crude product from the synthesis of norbornene, to a system based on two distillation columns; this system is a base system in which the norbornene is successively tailed and then topped.
However, the effectiveness of such a system is not very high because of the DCPD/CPD equilibrium. This is because the CPD formed in the first column during the heating of the charge is found in the top fraction. As the temperature of the second distillation column is lower, the reverse reaction results in a norbornene comprising residual heavy products, such as DCPD, DMON, and the like. Furthermore, the product obtained in this column bottom is coloured, which limits its applications.
French Patent Application FR-A-2,438,639 discloses a process for the manufacture of norbornene from the products of the reaction from the synthesis of norbornene between DCPD or CPD and ethylene:
in accordance with a first embodiment (illustrated by FIG. 1 of FR-A-2,438,639), the crude synthetic product is introduced into a first distillation column, the top product of which is, after condensation, reacted at 70xc2x0 C. with a mean residence time of one hour, in order to dimerize the CPD into DCPD, and is then reintroduced into the column. The mixture of the products which falls into the column bottom at 105xc2x0 C. is introduced into a second distillation column, the product comprising the norbornene being evaporated and all the heavy compounds being withdrawn as column bottom product;
in accordance with a second embodiment (illustrated by FIG. 2 of FR-A-2,438,639), the crude synthetic product is introduced into a first distillation column where, at 112xc2x0 C. and with a mean residence time of approximately 30 hours, a large portion of the product comprising the norbornene is evaporated, the heavy components, comprising a very high proportion of norbornene, being withdrawn as column bottom products. The top product is conveyed into a second distillation column where norbornene, still comprising dicyclopentadiene fractions, is obtained at the top and where norbornene of high purity is obtained at the bottom.
The disadvantage of the two systems described hereinabove, without topping, is the presence of light impurities, such as isoprene, resulting from the starting DCPD, which impurities are not separated from the norbornene. This presence of impurities necessitates the use of special grades of DCPD if it is desired to obtain norbornene of high purity.
In the processes according to U.S. Pat. No. 3,007,977 and FR-A-2,438,639, the purity obtained for the norbornene is a function not only of the reflux ratios and of the temperatures but also of the overall charge of the columns. This is because the monomerization of the DCPD is not immediate, so that the composition is also influenced by the residence times of the products in the columns.
An aim of the present invention is to provide a process for the purification of the norbornene by distillation which does not have any of the disadvantages of the processes of the prior art and which makes it possible to operate industrially for the production of a norbornene of satisfactory quality.
In accordance with the present invention, this aim is achieved, in other words a consistent quality of norbornene as far as possible free from residual heavy and light products can be obtained, without excessive loss of product, by a process involving three successive distillation columns.
The process according to the present invention for the purification of the norbornene obtained by the reaction of dicyclopentadiene or cyclopentadiene and ethylene, the crude reaction mixture comprising:
light impurities, the boiling temperatures of which are lower than that of norbornene;
medium-heavy impurities, the boiling temperatures of which are between that of norbornene and that of ethylnorbornene; and
heavy impurities, the boiling temperatures of which are greater than that of ethylnorbornene, the latter furthermore being included in the said heavy impurities, is characterized in that a first distillation of the crude reaction mixture is carried out in a tailing column (C1), removing a portion of the heavy impurities and a portion of the medium-heavy impurities; then a second distillation of the crude mixture, thus tailed, is carried out in a topping column (C2), removing the light impurities; and a third distillation of the mixture, thus topped, is subsequently carried out in a tailing column (C3), removing the remainder of the heavy and medium-heavy impurities.
Furthermore, the recombination of the cyclopentadiene to dicyclopentadiene, which begins in the tailing column (C1) and is continued in the topping column (C2), can be continued in a tank (R) interposed between the topping column (C2) and the second tailing column (C3).
In a particularly preferred way, the bottom flow from the second tailing column (C3) is recycled at the inlet of the first tailing column (C1).